I would remove the bubbler, it takes up a lot of free swimming space. Plus it was more for your entertainment anyways. There is no room in there for any of those decorations, by the way. How big is the skull thing in comparison to the betta log?
If it is as big or bigger than the log, I wouldn't put the skull in. To be honest, judging by the photo you definitely would have had room for a bigger tank. I would take the bubbler thing out now to make more room for your little guy. The only decor I could suggest for your tank is a few stones/marbles or some plants.
I agree with turtle10... the oxygen your betta gets is from breathing air from the surface and frequent water changes, in a tank that size...
The heater will take up some room as well. Be sure to measure the heater... make sure it will have enought room for water to circulate completely around it. Also, any items you add to the tank should not touch the heater.
Work out those two items first, before moving on to decorations, all then you'll really know how much room you have.
He would be happier with more swimming room. The shape of the tank is inconvenient for decorating. I suggest take the bubbler out to give the betta more room, as it is he already gets less than two gallons :/
I think for bettas, tanks that are longer are better than tanks that are taller. I would not put another decoration in there.